Sn=n²an ①a1+a2=4a2,a2=1/3S(n+1)=(n+1)²a(n+1) ②②-①:a(n+1)=S(n+1)-Sn=(n+1)²a(n+1)-n²an∴[(n+1)²-1]a(n+1)=n²an∴a(n+1)/an=n²/(n²+2n)=n/(n+2)∴n≥2时,a2/a1=1/3a...